Nekokino to Ame no Machi

One rainy day Amamiya came back to his apartment and found a strange mushroom growing on the floor. And when he tried to pick it up and throw away mushroom transformed into a girl with mushroom hat and cat ears. Apparently she is a catshroom. He doesn't like mushrooms but on the other had he likes cat ears. So what should he do? And why does this girl cling to the "mushroom" between his legs? (Source: MU)